"Evidently then," continued the professor, "the atoll is simply an annular terminal moraine of detritus shed alluvially into the sea, thus leaving a geosyncline of volcanic ash embedded with an occasional trilobite and the fragments of scoria, upon which we now stand."

Now, let us suppose, in the trough of the geosyncline, and upon the top of the normal layer, a deposit of, say, 10 kilometres (6.2 miles) of sediments is formed during a long period of continental denudation.